Seismic surveys image or map the subsurface of the earth by imparting acoustic energy into the ground and recording the reflected energy or “echoes” that return from the rock layers below. The source of the acoustic energy can be generated by explosions, air guns, vibrators, and the like. The energy source is positioned on or near the surface of the earth. Each time the energy source is activated, it generates a seismic signal that travels into the earth, is partially reflected, and, upon its return, may be detected at many locations on the surface as a function of travel time. The returning seismic energy is recorded as a continuous signal representing displacement, velocity, acceleration, or other recorded variation as a function of time. Multiple combinations of energy source and sensor can be subsequently combined to create a near continuous image of the subsurface that lies beneath the survey area. One or more sets of seismic signals may be assembled in the final seismic survey.
Technology continues to increase the resolution and complexity of seismic systems, such as high fidelity vibroseis systems. Vibroseis is a method used to propagate energy or a signal into the earth over an extended period of time as opposed to the near instantaneous energy provided by impulsive sources. The data recorded in this way must be correlated or inverted to convert the extended source signal into an impulse using the pilot sweep for the vibroseis method. In the case of ZenSeis® or HFVS, inversion is used with the measured ground motions of the vibrator or the best available proxy for that signal. The source signal using this method was originally generated by a servo-controlled hydraulic vibrator or shaker unit mounted on a mobile base unit, but electro-mechanical versions have also been developed.
A modern seismic vibrator typically comprises a hydromechanical or electromechanical system driven by a servo valve assembly that is electronically controlled through feedback loops. A baseplate is connected to a hydraulic or electromechanical system that raises and drops the baseplate to deliver a force into the ground. Since the actual force applied to the ground is known to differ from the pilot sweep, these techniques typically use the Ground Force Estimate put out by the controller as a proxy for the source wavelet.
Ideally, a vibrator used in vibroseis data acquisition should produce ground force as a known spatially-invariant wavelet such that any variations in refection data can be attributed to variations in geology, but due to the baseplate flexure and limitations of accelerometer placement on the baseplate, among other issues, the true ground force is not the same as the ground force estimate (GFE) used as the reference in calculating the cross-correlation function with the far-field data.
Actual field testing of conventional vibrators has shown that the ground force estimate (GFE) provided by the vibe electronics is wrong. In particular, it fails in terms of phase and frequency fidelity above about 50 Hz depending on the model of the vibrator. This appears to be due to poor assumptions built into the vibe controller and the ground coupling issues, hydraulic limits and baseplate flexure. The GFE is used in the separation step of ZenSeis® and other vibroseis systems to properly separate out the sources, such that any error in the GFE shows up as imperfect separation between the simultaneously acquired source records.
Conventional efforts to increase the recordable high frequency energy have been primarily focused on providing longer sweeps or lengthening the proportion of the sweep time for which the higher frequency energy is delivered into the ground. As a sweep-type vibrator delivers the seismic energy into the ground, it records each sweep and computes an approximate ground force delivered into the ground for use by a feedback circuit to control the vibe for the next sweep. This ground force approximation is used in subsequent analysis in seismic data processing.
Conventional vibrator technology uses a weighted-sum method to approximate the “ground force” during a sweep. In 1984, Sallas derived the weighted-sum method to approximate the true ground force (Sallas, 1984). The weighted-sum method assumes that a baseplate acts as a rigid body, and that a full coupling between the baseplate and the ground is achieved. Under these assumptions, the weighted-sum ground force is obtained by summing the weighted baseplate and reaction mass accelerations. The Sallas approximation or equation may be written as:−Fg=MrAr+MbAb,
where Mr=Mass of the reaction mass (kg); Mb=Mass of the baseplate (kg); Ar=Reaction mass acceleration (m/s); Ab=Baseplate acceleration (m/s); and Fg=Compressive force exerted on the earth by the baseplate (N). This is normally reported as the ground force of the vibrator.
The dynamics of vibrator system seems to inherently limit the power that is deliverable into the ground at high frequency. A low frequency is delivered by a longer, slower stroke of the reaction mass while a higher frequency stroke is fast and typically shorter in length.
While the Sallas approximation indicates that a fast stroke of shorter length provides equal force to the ground, the absence of the higher frequency data in the data traces or records from the field could mean that either the true force is not what is approximated by the Sallas equation or that consistent force across a broad frequency spectrum does not deliver consistent energy delivery across a broad frequency spectrum.
Different attempts have been made to increase the recorded bandwidth and for better source separation. For example, WO2005019865 describe a method for improving the efficiency of acquiring vibratory data with a method in which data from a number of vibrators shaking simultaneously in seismic proximity to one another are separated by using a number of phase encoded sweeps, where the number of sweeps is greater than or equal to the number of vibrators, resulting in a set of linear equations that can be solved simultaneously. However, the method does not exclude inherent errors resulted from poor GFE estimates.
U.S. Pat. No. 8,371,416 provides a method of obtaining True Ground Force by installing load sensors to the baseplate, so that the actual output can be measured as the basis for inversion. However, this represents significant capital cost to improve the existing equipment, and the durability of the load sensors may pose further problems. Also, the installation of load sensors further limits area available for imparting vibratory forces into the ground.
Therefore, there is the need for an efficient and reliable method of separating sources without the need to add on further equipment to existing system.